This invention relates to a process of forming a refractory metal thin film on a substrate by using a chemical vapor deposition (CVD), and more particularly to a process of forming a refractory metal thin film on a substrate with a good coverage of a surface of the substrate and without damage to the substrate.
In a production process for semiconductor devices, it is generally known that, when an aluminum-series material and a tungsten material are buried into a contact hole of an interlevel insulator film layer provided on the substrate, a titanium film layer is preliminarily formed at least on bottom and side surfaces of the contact hole.
The titanium film layer functions as a barrier metal layer for preventing an alloy reaction between aluminum and silicon when the aluminum-series material is buried into the contact hole. Whereas, the titanium film layer functions as a tight-contacting layer when the tungsten material is buried into the contact hole. However, the titanium film layer cannot function as a barrier metal layer nor a tight-contacting layer by itself though it is excellent in achieving a low ohmic contact. Therefore, the titanium film layer is generally used together with a titanium nitride film layer laminated thereover.
Meanwhile, an aspect ratio of the contact hole becomes increased in association with a high integration of semiconductor devices. Under this circumstance, it is essential that a vertical component of the particles spattered is strengthened by a collimated spattering method to form the above mentioned titanium film layer on an inside surface of the very small contact hole having as small a diameter as 0.25 .mu.m and as large an aspect ratio as 4 with a good coverage of its surface to be treated.
However, it is difficult to form the titanium film layer on a bottom surface of an extremely fine contact hole having an aspect ratio exceeding 5 even by the collimated spattering method. Consequently, an attempt has been recently made to apply a chemical vapor deposition (CVD) method exhibiting an excellent coverage upon the formation of the titanium film layer. It is expected that the titanium film layer or titanium silicide (TiSi.sub.x) film layer is formed by reduction of a halide gas, typically TiCl.sub.4, by using a hydrogen gas (H.sub.2) or a silane gas (SiH.sub.4). The formation of the titanium film layer or the titanium silicide film layer is, for example, based on the reduction reaction represented by the following chemical equation (1) in which a titanium film layer is produced by using a titanium tetrachloride gas (TiCl.sub.4) and a hydrogen gas (H.sub.2). EQU TiCl.sub.4 +2H.sub.2 .fwdarw.Ti+4HCl (1)
FIG. 1 shows, in section, one example of the prior semiconductor devices which has been actually produced by using the chemical vapor deposition method. In the prior semiconductor device, a semiconductor wafer is composed of a silicon substrate 101 and an interlevel insulator film layer 102 formed on the substrate. The semiconductor wafer is provided with a contact hole 103 formed through the interlevel insulator film layer 102. When such a semiconductor wafer with the contact hole 103 is subjected to the chemical vapor deposition method in which a titanium tetrachloride gas (TiCl.sub.4) and a hydrogen gas (H.sub.2) are used, there occurs a problem that the silicon substrate 101 is undesirably etched so that a corroded portion 104 is formed thereon. In this case, a deposited titanium film layer 105 is converted to a titanium silicide layer TiSi.sub.2 106 at a position where the titanium film layer is brought into contact with the silicon substrate 101.
The corroded portion 104 of the silicon substrate 101 is caused by reduction of TiCl.sub.4 not by H.sub.2 as represented by the above-mentioned equation (1) but by Si as represented by the following equation (2). EQU TiCl.sub.4 +Si.fwdarw.Ti+SiCl.sub.4 (2)
Since a bonding energy between a hydrogen atom and a chlorine atom is 431 kJ/mole and that between a silicon atom and a chlorine atom is 322 kJ/mole, it would be suggested that the reduction of TiCl.sub.4 is likely to be caused by the hydrogen gas (H.sub.2) rather than the silicon atom. Nevertheless, the reduction of the silicon substrate 101 is actually caused by the silicon atom. The reason therefor is considered as follows. That is, the reduction reaction is caused due to the fact that an adsorption probability of the TiCl.sub.4 gas to the silicon substrate 101 is higher than that of the TiCl.sub.4 gas to the hydrogen gas.
Particularly, in a case where a native oxide film remains non-uniformity on the silicon substrate 101, a reaction between the silicon substrate and the TiCl.sub.4 gas proceeds unevenly through a thin or lacking portion of the native oxide film. Since impurities are diffused into a portion of the silicon substrate 101 to which the contact hole 103 is opposed, the above-mentioned etching is drastically caused at the portion, which results in generating deficiencies such as an increased contact resistance and an increased leak current.
In addition, upon formation of other refractory metal film layers such as a tungsten film layer and a molybdenum film layer as well as the titanium film layer, there also occurs a similar problem concerning the undesirable etching of the silicon substrate 101. Further, when such a refractory metal film layer should be formed not only on the silicon substrate 101 but also on the aluminum-series material layer, identical deficiencies due to the etching are observed.